The present invention relates to a data transmission system, a communication station and a data transmission method and more particularly, to a communication technology which is suitable when digital signal data is transmitted between communication stations located away from each other in the form of microwave. Data to be transmitted include television signals of digital and analog types and other types of information. Explanation will be made below as to a general arrangement of a transmission system for transmitting data with use of high frequencies of about 1 GHz or more.
A transmission side once performs baseband modulation over input data to obtain a signal subjected to the baseband modulation, and creates a signal Sit having an intermediate frequency from the baseband-modulated signal. And the transmission side eventually creates a signal Sht having a final radio frequency (RF) after subjected to an up-conversion, and transmits the radio-frequency signal Sht. A receiver side, on the other hand, converts a radio-frequency signal Shr received and amplified in a similar manner to in the transmission side to a frequency down-conversion to obtain a signal Sir having an intermediate frequency, and then subjects the signal Sir to a baseband demodulation.
An example of a transmission system having a prior art arrangement is shown in FIG. 7.
In the transmission side, a baseband modulator 1 converts main data (to be transmitted) received from a terminal 22 to the signal Sit of the intermediate frequency on the basis of a clock CK2 of a frequency f2 received from an oscillator (OSC) 9B. In the illustrated example, the frequency fit of the signal Sit is related to the frequency f2 of the clock CK2. In other words, an accuracy in the frequency fit of the signal Sit depends on an accuracy in the frequency f2 of the clock CK2.
A frequency converter 3 converts with respect to frequency the signal Sit of the intermediate frequency fit to the signal Sht of the radio-frequency fht on the basis of a frequency f1 of a clock CK1 received from an oscillator (OSC) 9 as a reference frequency.
A power amplifier 4 amplifies the signal Sht to such an level that the signal Sht can have a predetermined transmission power, and transmits the power-amplified radio-frequency transmission signal from an antenna 5 toward an antenna 10 on the receiver side.
In the receiver side, a preamplifier 13 outputs a received signal as the radio-frequency-amplified signal Shr. A frequency converter 11 down-converts the signal Shr to the signal Sir of an intermediate frequency fir on the basis of a frequency f3 of a clock CK3 received from an oscillator (OSC) 9C as a reference frequency. In the illustrated example, a frequency shift in the oscillator (OSC) 9C is related to a frequency shift in the signal Sir of the intermediate frequency.
A baseband demodulator 12 demodulates the original main data from the signal Sir of the intermediate frequency on the basis of a frequency f4 of a clock CK4 received from oscillator (OSC) 9D as a reference frequency, and then outputs the demodulated data to a terminal 24.
The entire operation of the above system including the frequency conversion will now briefly explained with use of FIG. 8. It is assumed that the intermediate-frequency signal Sit on the transmission side has the frequency fit and the final radio-frequency signal Sht on the transmission side has the frequency fht. It is also assumed that the receive signal Shr on the receiver side has the frequency fhr and the intermediate-frequency signal Sir on the receiver side has the frequency fir.
In this case, the frequency conversion is carried out by multiplying the clock CK1 corresponding to the frequency difference f1 between the frequency fit and the reference frequency fht of the transmission side by the signal Sit.
This results in generation of the signal Sht of the frequency fht obtained by adding the frequency fit to the frequency f1 as well as generation of a signal Sxe2x80x2ht of a frequency fxe2x80x2ht obtained by subtracting the frequency fit from the frequency f1.
A specific example of how to obtain an final output frequency of 1.82 GHz will be explained below.
When fit=20 MHz and f1=1.8 GHz, output signals having frequencies of 1.82 GHz and 1.78 GHz are obtained. In this case, one of the output signals having the frequency of 1.78 GHz is removed by a filter and only the signal of the frequency of 1.82 GHz is used as a transmission signal.
Such a prior art arrangement as mentioned above has a practical problem that, even when the oscillators (OSC""s) 9 and 9C located as separated into the transmission and receiver sides are each of a high-accuracy crystal resonator type, differences in temperature characteristic, etc. between the oscillators causes the oscillation frequencies of the oscillators not to become exactly the same, because the oscillators are asynchronous, thus resulting more or less in a frequency difference.
As mentioned above, the intermediate-frequency signal Sit is up-converted by the frequency converter 3 of the transmission side to the radio-frequency signal Sht and is down-converted by the frequency converter 11 of the receiver side to the intermediate-frequency signal Sir.
At this time, if the frequency f1 of the oscillator (OSC) 9 coincides with the frequency f3 of the oscillator (OSC) 9C, then the frequency fit of the signal Sit of the transmission side coincides with the frequency fir of the signal Sir of the receiver side. However, due to the aforementioned frequency difference, since the signal Sir has such a frequency shift as mentioned above in the baseband demodulator 12 of the receiver side, the original main data cannot be normally demodulated.
In the case of ground wave digital TV broadcasting using an orthogonal frequency division multiplex (OFDM) system for example, a baseband modulating and demodulating system uses about 6,000 multicarriers having an interval of about 1 KHz. Thus when there exists 1 KHz or more of accuracy lack, in particular, in the frequency conversion of the receiver side, this will involve great influence such as difficult normal demodulation.
In the case of OFDM wave used in the ground wave digital TV broadcasing, there is a pilot signal interpolation system wherein some of multicarriers are used as frequency references in transmission and receiver sides. A technique for interpolating a pilot signal in an OFDM signal is disclosed in xe2x80x9cDevelopment of the OFDM Modemxe2x80x9d, Kisoda. et al., Technical Report of The Institute of Image Information and Television Engineers, Aug. 26, 1997, pp. 13-18.
However, the pilot signal interpolation system has its limit in synchronizable range. In particular, when signals of an UHF band are frequency-converted to signals of a microwave band as a transmission band and converted again to signals of the UHF band in the receiver side, the frequency shift becomes great.
In this case, the accuracy of the oscillator is defined by ppm. For example, even when the oscillators of the transmission and receiver sides have each an accuracy of 1 ppm, a frequency error between the transmission and receiver sides is 7 KHz for a transmission band of 7 GHz and is about 800 Hz for a transmission band of 800 MHz.
Accordingly when frequency conversion is carried out from the UHF band to the microwave band, an error caused by the up-conversion/down-conversion of at the transmission/receiver side is increased to about 10 times an error of the frequency conversion in UHF band, which causes a great harm in the entire baseband demodulating operation based on the pilot signal interpolation system.
Further, even when the frequency shift is suppressed to such a level as to cause no influence on the baseband demodulation, the frequency error to be transmitted in broadcasting applications is restricted to an accurate range of several tens of Hz.
For this reason, an arrangement of provision of accurate and highly expensive oscillators of rubidium or the like in the respective frequency converters in the transmission and receiver sides has had to be employed for the broadcasting applications.
Further, in addition to the problem of oscillator frequency fluctuations, the data transmission system has a problem that there must be provided a private line for maintenance contact or control signal which is to be used by a radio facility manager, in addition to the main data transmission line.
More specifically, there are transmission requests which include a digitized voice signal for contact between the transmission and receiver sides and control data for control of devices in the receiver or relay stations. In order to meet such a demand, it is indispensable as a practical matter to secure a special channel for the control data in addition to the main data line.
For example, portable phones spread in these years, but relay or transmission points of the phones are often positioned out of their speech enable areas, which requires provision of additional radio channel exclusive for maintenance contact and control signals. In reality, however, it is practically impossible to provide such radio channels as contactable with all the relay and transmission points.
There is also considered a method for inserting contact information in part of the aforementioned main data (such as a vertical blanking period having VITC (Vertical Interval Time Code) or the like embedded therein in an analog system) in the data transmission. In this method, however, when it is desired to make a separate contact from a first relay point to a second relay point, it becomes highly troublesome to exchange detailed contact information.
More specifically, in order to interchange information inserted in the main data, it is necessary to demodulate the main data up to its baseband, interchange part thereof for other data, and then re-modulate it. This involves generation of a processing delay time. Accordingly it is impossible to interchange the partial data during use of the main data as ordinary broadcasting data.
It is therefore an object of the present invention to solve the problems in the prior art, enable elimination of a frequency shift even when an inexpensive oscillator is used and normal demodulation of predetermined data, and also enable transmission of other data in addition to the predetermined data.
In a data transmission system in accordance with the present invention, a transmission side adds to a main signal a sub-signal having a frequency out of a frequency band of the main signal as frequency conversion information and transmits the composite signal including the sub-signal and the main signal, whereas, a receiver side extracts the added sub-signal from the received composite signal to make a frequency conversion reference for demodulation of the main signal coincide with that of the transmission side. At the same time, the transmission side modulates the sub-signal to be added with sub-data and also transmits it, while the receiver side demodulates the sub-data.